Slashing bread before baking it is essential. It not only is decorative but it also helps the bread to expand fully and evenly, improving the bread's crumb structure and ensuring that the bread expands symmetrically.
Long slashes across long breads are ususally made about ¼ inch deep with the razor held at an angle to the dough, about 30°, creating more of a flap under the skin of the dough than a gash straight into the dough. On round breads, the razor is often held vertically, although not always.
Take your time making slashes; they need not to be done with professional speed. If you feel that a slash it not deep enough, just go over it again until you are pleased with it.
After baking, you will have wonderful bread with a beautiful grigne.